Many chemical and evaporation process operations involve molecular mass transfer between a gas stream and a liquid stream across a gas-liquid interface. For example, in gas absorption operations, soluble components of a gas stream are transferred to and dissolved in a liquid stream, and in desorption, or stripping, operations, volatile components from a liquid stream are transferred from the liquid to a gas stream.
When material is transferred from one bulk phase to another bulk phase across the interface which separates the two phases, the resistance to mass transfer in each phase causes a concentration gradient to occur in each phase. As the transferring component moves across the phase interface, the component concentration in the phase from which the component transfers decreases at the interface, creating a concentration gradient as compared to the bulk concentration of that phase. The phase into which the component transfers then has a higher concentration of the transferring component near the interface compared to the bulk concentration of that phase. The concentration at the interface of the transferring component in the liquid or gaseous phase is generally unequal even if expressed in the same concentration units. The relative concentration in each phase is described by the laws of thermodynamic equilibrium.
In phase equilibria it is generally recognized that the concentration in the gaseous phase is related to the concentration in the liquid phase by a partition coefficient or equilibrium factor. Gas or vapor versus liquid or solvent phase equilibrium data have been studied and recorded for many chemical compounds in various solvents as a function of concentration, temperature, and pressure. Also phase equilibrium behavior for complex mixtures of compounds in various solvents and solvent mixtures at various temperatures and pressures has been reported.
In general, the partitioning of a component between the vapor and liquid phases can be expressed by the following equation; EQU Y.sub.A =K.sub.A X.sub.A
where
Y.sub.A =mole fraction of component A in the vapor phase PA1 X.sub.A =mole fraction of component A in the liquid phase PA1 K.sub.A =equilibrium or partition coefficient for component A between the gas and liquid phases. PA1 N.sub.A =molar flux (moles/L.sup.2 t) (L=length; t=time) PA1 D.sub.AB =binary diffusivity for system A-B (L.sup.2 /t) PA1 C.sub.AO =the interfacial concentration of A in the liquid phase, which is assumed to be at equilibrium with the gas phase at the interface (moles/L.sup.3) PA1 .alpha.=film thickness (L) PA1 R=C.sub..alpha. /C.sub.AO PA1 C.alpha.=concentration in the main body of the liquid.
It is generally assumed that mass transfer between a volume of liquid and a volume of gas across an interfacial contact area is instantaneous, but the actual rate of transfer is subject to various limiting factors, including the rate of diffusion of component molecules through the liquid to or from the interface, and the rate of diffusion of component molecules through the gas from or to the interface. It is also generally considered that the interfacial transfer itself is sufficiently rapid to be considered instantaneous. Although in most cases the rate of diffusion through the liquid is a significant factor, transfer between liquid and gas is always favored by maximizing the interfacial area relative to liquid and gas volumes and by minimizing the distance of diffusion through the liquid to the interface.
The dynamics of mass transfer across a gas-liquid interface may be quantified. The rate of transfer of a compound at the gas-liquid interface has been derived by Bird, Stewart, and Lightfoot, and expressed as EQU N.sub.A =(D.sub.AB C.sub.AO /.alpha.)(1-r)
where
The foregoing equation is taken from the book Transport Phenomena, R. Byron Bird, Warren E. Stewart, and Edwin N. Lightfoot, John Wiley & Sons, Inc., 1960 (at page 535).
Traditional methods of creating conditions for interfacial mass transfer include the use of simple aerated tanks, spray towers, bubble tray columns, and packed columns to create a gas-liquid interface. Traditional technology uses counter current, multiple equilibrium stages in order to take full advantage of the equilibrium conditions for the component to be transferred. Generally, the design of a tray or packed tower absorption or stripping column will incorporate a number of transfer units (theoretical plates) in series in order to achieve the desired final transfer performance. Within each theoretical plate or transfer unit the mass transfer dynamics attempt to reach equilibrium, based upon relative volumes of gas and liquid between which transfer may occur.
While these traditional methods and associated apparatus do achieve mass transfer, they are inefficient, requiring long processing times, large equipment volumes, and high overall gas to liquid volumetric flow ratios. The inefficiency associated with the traditional prior art approaches arises largely from the relatively low gas-liquid interfacial area to volumes provided by the equipment, and the relatively long liquid diffusion distances to an interface. As a result of the inefficient gas to liquid contact, the internal apparatus volume required for each transfer unit (i.e., the volume required for mass transfer equilibrium to be reached between given volumes of gas and liquid under established operating conditions) is large. With regard to traditional apparatus, comparisons are typically expressed in terms of the height of a transfer unit.
With traditional apparatus, such as tray towers and packed towers, the designer's ability to increase the rate of mass transfer within the apparatus (and thus decrease the apparatus volume for each transfer unit) by increasing the volumetric flow ratio of gas to liquid is limited by the need to avoid foaming and liquid entrainment in the gas stream.
It has been suggested that improved mass transfer performance in an operation for the gas stripping of volatile contaminants from a liquid stream may be achieved through the use of an air sparged hydrocyclone similar to designs used in the mineral processing industry for separation of solid particles from an aqueous suspension. Examples of particle separation methods and apparatus may be found in U.S. Pat. Nos. 4,279,743; 4,397,741; 4,399,027; 4,744,890; 4,838,434; and 4,997,549. Each of these references is specifically based upon the concept of passing bubbles of air through a suspension of solid particles so that hydrophobic particles attach to air bubbles and form a cohesive froth that may be removed from the apparatus. No interphase mass transfer is involved, and neither the method nor the apparatus design are concerned with the creation of gas-liquid contact conditions favorable for efficient interphase transfer. The conditions taught by those reference patents, characterized by significant foaming and liquid entrainment, are more characteristic of flooding conditions in tray towers or packed towers used for an interphase transfer operation.
In a 1993 paper ("A Novel High-Capacity Technology for Removing Volatile Organic Contaminants From Water", Proceedings of Waste Stream Minimization and Utilization Innovative Concepts--An Experimental Technology Exchange, Ye Yi, April, 1993) the use of an air sparging process and apparatus was proposed for an interphase mass transfer operation involving removal of volatile components from a liquid stream. More specifically, the paper proposed a continuous process in which contaminated water would be introduced into the interior of a porous tube in a swirl flow pattern and air would be introduced through the porous tube into the water flow. The porous tube was described as disposed in a vertical orientation and the contaminated water would be pumped into the tube at the top and allowed to swirl around the inner wall of the tube to the bottom, while air would be forced through the tube into contact with the water. The total air to water volumetric flow ratio of two (2) was disclosed in the paper for a single pass through the apparatus. The paper further taught that it will be necessary for the water to be recycled through the apparatus a number of times to achieve significant transfer performance.
Although the method parameters and apparatus design suggested by Yi appear to offer the potential for achieving an incremental improvement in the creation of gas-liquid contact conditions for interphase mass transfer, the approach remains well short of optimal, and the mass transfer efficiencies available remain low.
There remains a substantial and unfilled need for a method of optimizing gas-liquid interfacial contact conditions for efficient interphase mass transfer, and a need for apparatus in which optimal conditions can be economically created, controlled, and maintained.